1. Field of the Invention
The present invention relates to a high breakdown voltage push-pull circuit for a semiconductor device, and more specifically to a high breakdown voltage push-pull circuit for a semiconductor device used for IC or LSI using a high supply voltage.
2. Description of the Prior Art
Conventionally, when transistor elements used for a high breakdown voltage push-pull circuit for a semiconductor device are selected, the transistor elements have been so far selected according to the maximum breakdown voltage of the high breakdown voltage push-pull circuit. In other words, the transistor elements having a collector-emitter breakdown voltage V.sub.CEO higher than the maximum supply voltage to be used have been so far used. For instance, when a supply voltage of 12V to 18V is to be used for the high breakdown voltage push-pull circuit for a semiconductor device, the transistor elements having a collector-emitter breakdown voltage V.sub.CEO of 18V have been so far selected.
In general, the breakdown voltage of the transistor elements is roughly proportional to the size thereof. Therefore, when the transistor element having a high breakdown voltage is selected, since the area of the transistor element itself is large, there exists a problem in that the chip size thereof also increases. As well understood by FIGS. 5(a) and 5(b), for instance, the area of the 18V breakdown voltage transistor element is about 3.3 times larger than that of the 8V breakdown voltage transistor element. Therefore, as a matter of course, when the number of transistor elements is the same in both, a difference in transistor size between both causes a difference in chip size between both.
Further, in this case, it is seldom that 18V breakdown voltage is required for all the transistor elements. In general, the number of transistor elements required for 18V breakdown voltage is about 10% of the total number of the transistor elements, so that 8V breakdown voltage transistor elements can be used as the remaining 90% transistor elements. However, it is extremely difficult to mix both the 18V breakdown voltage transistor elements and the 8V breakdown voltage transistor elements together during the manufacturing process. This is because there exists a difference in basic structure between both, for instance such as a depth of a diffusion layer required to form the transistor element. In other words, when the depth of the diffusion layer is fitted to the basic structure of one transistor element (e.g., 8V), since the depth thereof does not fit to the other transistor element (e.g., 18V), it is impossible to manufacture both the transistor elements by the same manufacturing process.